I'm guessing that the 1200 that you have may be configured from the factory to put out more GPS information and at a speed that the older PF3000 cannot digest. At least this is the situation with some combinations where a newer GPS is used with the older PF3000. Usually the newer GPS can be reconfigured to work with the PF3000. The PF3000 doesn't have the capability to reconfigure the newer GPS itself since the newer GPS didn't exist at the time the PF3000 was produced.

Sometimes a laptop with a special program is used. Other times, temporarily connecting the newer GPS to a newer display will allow a person to reconfigure the GPS via the newer display. Once the GPS is reconfigured to settings that are acceptable to the PF3000, it can be removed and connected to the PF3000. The newer settings remain and it can be reinstalled and work with the PF3000.

As I recall the PF3000 wants settings of 4800, 8,N, 1 with the GGA and VTG messages ON and all others OFF.

Your results as far as wandering may not be any better. I wouldn't be too concerned about that for harvesting. I use a GPS 6500 with TerraStar correction in the planting tractor but use an older Raven Phoenix 200 on WAAS in the combine. When doing some split planter analysis, I do notice my harvest track does not line up perfectly with the planted track as the different color bands for the varieties are not always completely "walked over" as I combine. Since the combine has a 20' corn head, I'm well within the correct variety so this has no effect on my analysis.